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CDC Flu Update: Activity Continues to Decline

Mar 20, 2017

According to the CDC's FluView report for the week ending March 11, 2017 (week 10), flu activity remains elevated, but continues to decline. The 2016-2017 flu season has likely peaked and is now on the decline, however 36 states continue to report widespread flu activity and the southern and southeastern parts of the US are still experiencing high levels of influenza activity. While activity is declining, there are likely still several weeks of flu activity still to come this season. Influenza A (H3N2) viruses have been most common overall this season and they continue to predominate, but there has been an increasing proportion of influenza B viruses detected in recent weeks. Based on interim estimates, flu vaccines this season have reduced a vaccinated person’s risk of getting sick and having to go to the doctor because of flu by about half (48%). CDC recommends annual flu vaccination for everyone 6 months of age and older. Anyone who has not gotten vaccinated yet this season should get vaccinated now.

Laboratory Data: ?Nationally, the percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories during the week ending March 11 was 18.3%.
?Regionally, the three week average percent of specimens testing positive for influenza in clinical laboratories ranged from 8.4% to 27.3%.
?During the week ending March 11, of the 4,967 (18.3%) influenza-positive tests reported to CDC by clinical laboratories, 3,037 (61.1%) were influenza A viruses and 1,930 (38.9%) were influenza B viruses.
- The most frequently identified influenza virus type reported by public health laboratories during the week ending March 11 was influenza A viruses, with influenza A (H3N2) viruses predominating.
- During the week ending March 11, 514 (70.7%) of the 727 influenza-positive tests reported to CDC by public health laboratories were influenza A viruses and 213 (29.3%) were influenza B viruses. Of the 492 influenza A viruses that were subtyped, 476 (96.7%) were H3N2 viruses and 16 (3.3%) were (H1N1)pdm09 viruses.
- Since October 1, 2016, antigenic and/or genetic characterization shows that the majority of the tested viruses remain similar to the recommended components of the 2016-2017 Northern Hemisphere vaccines.
- Since October 1, 2016, CDC tested 1,745 specimens (195 influenza A (H1N1)pdm09, 1,126 influenza A (H3N2), and 424 influenza B viruses) for resistance to the neuraminidase inhibitors antiviral drugs. None of the tested viruses were found to be resistant to oseltamivir, zanamivir, or peramivir.

Influenza-like Illness Surveillance: For the week ending March 11, the proportion of people seeing their health care provider for influenza-like illness (ILI) for influenza-like illness (ILI) was 3.7%. This remains above the national baseline of 2.2%. Seven regions (Regions 1, 3, 4, 5, 6, 7 and 8) reported ILI at or above their region-specific baseline level. For the last 15 seasons, the average duration of a flu season by this measure has been 13 weeks, with a range from one week to 20 weeks.

Geographic Spread of Influenza Viruses: Widespread influenza activity was reported by 36 states (Alabama, Alaska, Arizona, Arkansas, California, Connecticut, Delaware, Florida, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Vermont, Virginia, West Virginia, and Wisconsin). Regional influenza activity was reported by Guam, Puerto Rico and 11 states (Arizona, Colorado, Georgia, Idaho, Illinois, Montana, South Dakota, Texas, Washington, West Virginia, and Wyoming). Local influenza activity was reported by the District of Columbia and three states (Hawaii, Oregon, and Utah). No influenza activity was reported by the U.S. Virgin Islands. Geographic spread data show how many areas within a state or territory are seeing flu activity.

Flu-Associated Hospitalizations: Since October 1, 2016, a total of 13,133 laboratory-confirmed influenza-associated hospitalizations have been reported. This translates to a cumulative overall rate of 33.7 hospitalizations per 100,000 people in the United States. This translates to a cumulative overall rate of 46.9 hospitalizations per 100,000 people in the United States. This is higher than the hospitalization rate at this time (40.4 per 100,000) during the 2012-2013 flu season, when influenza A H3N2 viruses also predominated.

- The 2012-2013 flu season was a recent influenza A H3N2-predominant season in the United States. Influenza A (H3N2) viruses also predominated during the 2014- 2015 flu season, but more than 70% of the H3N2 viruses circulating that season were different or "drifted" from the H3N2 vaccine virus.
- The hospitalization rate among people 65 years and older is 214.0 per 100,000. This is the highest rate of any age group. The hospitalization rate for people 65 and older for the same week during the 2012-2013 flu season was 173.6 per 100,000.
- The hospitalization rate among adults 50-64 years is 45.7 per 100,000. During 2012-2013 flu season, the hospitalization rate for that age group for the same week was 37.3 per 100,000 respectively.
- The hospitalization rate among children younger than 5 years is 31.3 per 100,000. During 2012-2013 flu season, the hospitalization rate for people in that age group for the same week was 58.5 per 100,000 respectively.
- During most seasons, children younger than 5 years and adults 65 years and older have the highest hospitalization rates. ?Hospitalization data are collected from 13 states and represent approximately 9% of the total U.S. population. The number of hospitalizations reported does not reflect the actual total number of influenza-associated hospitalizations in the United States. Additional data, including hospitalization rates during other influenza seasons, can be found at http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.

Mortality Surveillance: The proportion of deaths attributed to pneumonia and influenza (P&I) was 8.0% for the week ending February 25, 2017 (week 8). This percentage is above the epidemic threshold of 7.5% for week 8 in the National Center for Health Statistics (NCHS) Mortality Surveillance System. The weekly percentage of deaths attributed to P&I has exceeded the epidemic threshold for eight consecutive weeks this season.

Pediatric Deaths: ?Five influenza-associated pediatric deaths are being reported by CDC for the week ending March 11, 2017.
- Two deaths were associated with an influenza A (H3) virus and occurred during weeks 7 and 8 (the weeks ending February 18, and February 25, 2017, respectively)
- One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 4 (the week ending January 28, 2017).
- Two deaths were associated with an influenza B virus and occurred during weeks 8 and 9 (the weeks ending February 25, and March 4, 2017).
- A total of 53 influenza-associated pediatric deaths have been reported for the 2016-2017 season.
- Additional information on pediatric deaths for the 2016-2017 season is available on FluView Interactive at: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.